The present invention relates generally to an optical disc in which servo areas each having recorded therein servo pits including at least clock synchronizing information are periodically disposed in each of tracks as well as an optical disc system for reading data or information from the optical disc. More particularly, the present invention is concerned with the optical disc and the optical disc reading or reproducing apparatus in which a high-speed access operation can be performed accurately by detecting the number of tracks traversed by a light spot.
Heretofore, various approaches or means for allowing the high-speed access operation in optical disc systems such as CD players, optical video disc systems or the like have been proposed. As one of such proposals, there is known a method which relies on the use of an external scale. According to this method, the position of an optical head is detected by means of the external scale, wherein the travel of the light spot projected from the optical head toward a desired destination track (i.e. the access operation) is controlled on the basis of the result of the detection. This method however suffers from many problems in that a high-precision external scale is required, correction is necessary for eccentricity of the tracks on the optical disc, rounding errors are likely to occur due to a rough scale pitch of the external scale and others. Thus, with this method, it is impossible to perform the access operation on a track-by-track basis. Under the circumstance, this method is employed solely for a rough access operation while a fine access operation as required is separately performed by resorting to another means. Thus, much time is involved in the access operation, giving rise to an additional problem.
On the other hand, there is known another method according to which the number of tracks on the optical disc traversed by the light spot is detected, wherein on the basis of the results of the detection, the position of the light spot as well as the speed at which the light spot is moved (hereinafter referred to as the moving speed of the light spot) is determined for allowing the light spot to be moved to a desired destination track (also referred to as the desired track) by controlling the moving speed of the light spot.
As one of the methods for detecting the number of the tracks traversed by the light spot, known is a method according to which guide grooves known as pre-grooves formed previously on the optical disc for the purpose of performing the tracking control of the light spot upon data reproduction or reading from the optical disc are made use of. (In this conjunction, reference may be made to, for example, a thesis entitled "High-Speed Access System for Optical Disc Memory" presented at the Optical Memory Symposium '86 held on Dec. 18, 1986.) More specifically, the optical disc is previously formed with pre-groove(s) in a helical or concentrical pattern, wherein a tracking control signal is derived from the pre-groove to be utilized for performing the tracking control of the light spot so that the light spot is moved along the pre-groove while allowing the data to be recorded or reproduced on or from the track by the light spot. This pre-groove is formed continuously so that each track is associated with the pre-groove.
Thus, whenever the light spot traverses the pre-groove, the associated track is necessarily traversed by the light spot. Accordingly, by detecting a change in amplitude of a signal outputted from a photodetector which change is brought about when the pre-groove is traversed by the light spot and by counting the occurrences of such change, it is possible to know the number of the tracks which has been traversed by the light spot.
On the other hand, there is another known example of the tracking control system a so-called sample servo system in which regions or areas are provided discontinuously or discretely on the optical disc for detecting the tracking control signal.
An exemplary structure of the optical disc in which the sample servo technique is adopted is illustrated in FIG. 1 of the accompanying drawings. As can be seen in this figure, servo areas A.sub.S are provided periodically at a predetermined distance or interval on each of tracks 3a, 3b, 3c, 3d, and so forth formed in the optical disc, wherein in each of the servo area A.sub.S, a pair of servo pits 1a and 1b are recorded on the disc with an equal distance from the center line of the track (indicated by broken lines) in opposite directions therefrom while being concurrently deviated or offset from each other in the longitudinal direction of the track Now assuming that the optical disc of such structure is rotated in the direction indicated by an arrow X and that a light spot 4 is currently located on the track 3a, then the servo pit 1a in the servo area A.sub.S is first detected by the light spot 4, being followed by detection of the servo pit 1b. Thus, by comparing the amplitude of the signal resulting from the detection of the servo pit 1a with that originating in the servo pit 1b, there can be derived or obtained a tracking control signal.
Parenthetically, the servo area A.sub.S has recorded therein a synchronizing pit for the purpose of clock synchronization. However, since the synchronizing pit is irrelevant to the essence of the subject matter of the invention, description thereof is omitted herein.
The sample-servo type optical disc of the structure described above is certainly advantageous in that the provision of a pre-groove which is required to have uniform width and depth with a high precision is rendered unnecessary, which in turn means that interference otherwise taking place between the control information reproduced from the pre-groove and the data information read from the track can no longer present any problem. However, there arises another problem in this known servo-sample type optical disc in conjunction with the high-speed access operation. More specifically, traversing of the tracks by the light spot 4 is detected in terms of detection of traversing of the servo areas A.sub.S on the track by the light spot 4. However, since the servo areas A.sub.S are discontinuously provided on the track with a data area being interposed between the adjacent servo areas, it may frequently occur that the light spot 4 traverses the track not exactly at the servo area but between the servo areas. In that case, it is impossible to detect the number of tracks traversed by the light spot without resorting to some other measures.
A method of making it possible to detect the number of tracks traversed by the light spot in the servo type optical disc is disclosed, for example, in "SPIE, Vol. 695: optical Mass Data Storage II" pp. 160-164. According to this known method, a gray code or the like representing a track address is recorded for each of the tracks. In this known system, there may sometimes occur a situation wherein the track portion where the above-mentioned code is not recorded is traversed by the light spot moved at a high speed in the radial direction in the course of high-speed access operation. However, once any one of the abovementioned codes is detected as the result of traversing of the track area having recorded therein that one code by the light spot, it is then possible to determine the position of the light spot on the basis of the code traversed by the light spot and the code recorded on the track from which the high-speed access operation is started.